It is a well known fact that lithium-ion secondary batteries, which have high energy density and show no significant decrease in discharge capacity, have been used for a power source of mobile tools, such as mobile phones and laptop computers. In recent years, with mobile tools becoming smaller, there is a need for smaller lithium-ion secondary batteries for use therein. Further, with the development of hybrid cars, solar power generation, and other technologies from the viewpoint of preventing global warming, etc., the application of super capacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, has been expanding at an accelerated pace, and there is a need for a further increase in energy density.
An electrical storage device, such as a lithium-ion secondary battery or a super capacitor, is configured, for example, such that a positive electrode, a negative electrode, and a separator made of polyolefine or the like between them are arranged in an organic electrolytic solution containing as an electrolyte a fluorine-containing compound, such as LiPF6 or NR4.BF4 (R is an alkyl group). The positive electrode includes a positive electrode active material, such as LiCoO2 (lithium cobalt oxide) or active carbon, and a positive electrode current collector, while the negative electrode includes a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector. With respect to their shape, generally, a current collector having applied thereon an active material is formed into a sheet. The electrodes are each subjected to a large voltage and also immersed in a highly corrosive organic electrolytic solution that contains a fluorine-containing compound. Accordingly, materials for a positive electrode current collector, in particular, are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, nearly 100% of the time, the material for a positive electrode current collector is aluminum, which is a good electrical conductor and also forms a passive film on the surface to offer excellent corrosion resistance (as materials for a negative electrode current collector, copper, nickel, and the like can be mentioned). An aluminum foil having a thickness of about 15 to 20 μm that is produced by rolling, generally, is used as a positive electrode current collector.
However, while the passive film that is formed on the surface of an aluminum foil contributes to suppress the corrosion of the positive electrode current collector, there is such a problem that the film blocks electrical conductivity between the positive electrode current collector and the positive electrode active material and brings about the increase in internal resistance of an electrical storage device caused by the increase in the surface resistance (interface resistance) of the positive electrode current collector. This problem causes heat generation or lowering of charge/discharge efficiency of the electrical storage device, and, as the result, is an obstacle to higher energy density of the electrical storage device.
As a helpful technology for solving the above-mentioned problem, for example, Patent Document 1 discloses an electrode member for solid electrolytic capacitor, which is produced by a process comprising the steps of adhering a carbon-containing material to the surface of aluminum, and heating the aluminum with which a carbon-containing material has been adhered to the surface thereof in a state of being arranged in a space containing a hydrocarbon-containing material, and has a carbon-containing layer formed on the surface of aluminum via an interstitial layer containing an aluminum element and a carbon element. Patent Document 2 discloses an electrode for nonaqueous electrolyte battery that is provided with a current collector including an aluminum foil or an aluminum alloy foil, and an active material-containing layer that is formed on the surface of the current collector and comprises an active material, an electrical conductive agent containing a carbon particle group, and a binding agent. These technologies can be rated as a method for reducing the surface resistance of an aluminum foil, however, there is such a problem that the production process becomes complex, in order to provide another layer on the surface of an aluminum foil. Further, there is also such a problem in a method of forming another layer on the surface of an aluminum foil that it cannot achieve the thickness reduction of a positive electrode current collector for size reduction or higher energy density of electrical storage devices.